The present invention relates, in general, to a pressure measuring device for an injection molding machine.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
A typical injection molding machine includes an injection mold and an injection unit for injecting a shot of plastic melt into the injection mold. The injection unit is provided with a rotary drive to rotate a plasticizing screw for advancing plastic in a barrel while being plasticized, and an injection drive or thrust generator for axially moving the screw to thereby inject the shot into the adjacent injection mold. In order to measure the thrust applied on the plasticizing screw by the injection drive for determining the melt pressure of the plastic melt in the barrel, the use of a load detector has been proposed. Examples include European Patent specification EP 0 350 872 B1, U.S. Pat. Nos. 5,206,034 and 6,247,913. The load detector is hereby positioned in the drive train between the injection drive and the plasticizing screw and includes a force transmitting element, which elastically deforms when being subjected to the thrust, and a pickup device for measuring the deformation. Conventional load detectors suffer shortcomings because the force transmitting element is realized in the form of a separate mechanical and fairly expensive precision structure and because the integration of the measuring device inside the injection unit so that a replacement is only difficult to implement and time-consuming. When the load detector is placed directly in the driveshaft, which is coupled with the plasticizing screw, in the shaft region between driveshaft bearing and the plasticizing screw, the supply of auxiliary energy and the transmission of the measuring value is difficult to implement between the part that conjointly rotates with the driveshaft and the stationary part of the load detector.
It would therefore be desirable and advantageous to provide an improved pressure measuring device for an injection molding machine to obviate prior art shortcomings and to attain high precision in measurement while being simple in structure and reliable in operation.